Can we characterize A-P/IAP behavioural phenotypes in people with chronic pain?

Two behavioural phenotypes in healthy people have been delineated based on their intrinsic attention to pain (IAP) and whether their reaction times (RT) during a cognitively-demanding task are slower (P-type) or faster (A-type) during experimental pain. These behavioural phenotypes were not previously studied in chronic pain populations to avoid using experimental pain in a chronic pain context. Since pain rumination (PR) may serve as a supplement to IAP without needing noxious stimuli, we attempted to delineate A-P/IAP behavioural phenotypes in people with chronic pain and determined if PR can supplement IAP. Behavioural data acquired in 43 healthy controls (HCs) and 43 age-/sex-matched people with chronic pain associated with ankylosing spondylitis (AS) was retrospectively analyzed. A-P behavioural phenotypes were based on RT differences between pain and no-pain trials of a numeric interference task. IAP was quantified based on scores representing reported attention towards or mind-wandering away from experimental pain. PR was quantified using the pain catastrophizing scale, rumination subscale. The variability in RT was higher during no-pain trials in the AS group than HCs but was not significantly different in pain trials. There were no group differences in task RTs in no-pain and pain trials, IAP or PR scores. IAP and PR scores were marginally significantly positively correlated in the AS group. RT differences and variability were not significantly correlated with IAP or PR scores. Thus, we propose that experimental pain in the A-P/IAP protocols can confound testing in chronic pain populations, but that PR could be a supplement to IAP to quantify attention to pain.

Pain intensity ratings of the experimental stimulus intensity were checked and re-calibrated between runs of the behavioural experimental pain tasks if needed.

Transcutaneous Electrical Stimulation (TENS) Device Calibration
For each participant we established the electrical stimulus voltage (mV) that elicits a pain intensity score of approximately 50/100 (0-no pain, 100-worst pain imaginable) to be used in the behavioural tasks. Electrodes from the transcutaneous electrical nerve stimulation (TENS) device were placed on the participants' left volar forearm to stimulate the median nerve at 50Hz. 1 Stimulus intensity was slowly increased in increments of 0.5 millivolts (mV) until the participant verbally reported a pain intensity level between 40-60/100. This was repeated 3 times with a 1-minute interval between repetitions. We also checked that the stimulus intensity would provide the desired pain intensity over a sustained period similar to what would be used in the behavioural test. Thus, participants used a trackball to provide continuous ratings of pain intensity evoked by 20s of TENS at the voltage level established during the calibration procedure. This was repeated twice, with 30 seconds intervals between each repetition. 2

Pain Scores of the Ankylosing Spondylitis (AS) Group
The AS participants completed the painDETECT questionnaire, which is a screening-tool that can be used to detect the likelihood of neuropathic pain. 3,4 This questionnaire includes questions that assess current chronic pain at the time of testing and average pain over the past 4 weeks on a scale from 0 to 10 (0 =no pain, 10 = max pain). The group mean pain (+/-SD) at the time of testing was 2.2 +/-2.0 and the average pain score over 4 weeks was 3.3 +/-2.3. There were no significant differences in current pain scores at the time of testing between A-types and P-types (Mann-Whitney U test: Atypes: M=2.0, SD=2.0, P-types: M=2.6, SD=2.0) (p=0.24, Cohen's d=0.32). There was only a marginally significant difference in average pain scores over 4 weeks between A-types and P-types, such that P-types had overall higher pain-scores than the A-types (A-types: M=3.0, SD=2.5, P-types: M=4.2, SD=1.6) (p=0.056, Cohen's d=0.53).

Sex Difference Analyses
We examined the sex differences in the reaction time (RT) means and variances (RTv), and the intrinsic attention to pain (IAP) scores using independent-sample t-tests for the HCs and the AS group. We also examined the difference between PCS-R scores using a Mann-Whitney-U test between males and females for the HCs and the AS group.

Intrinsic Attention to Pain and Rumination Scores
There were no significant sex differences in IAP scores in HCs (t=0.68, p=0.50, Cohen's d=0.